ZigBee™ : An Overview

ZigBee is a wireless network standard defined by the ZigBee Alliance. The ZigBee Alliance is an organization with a mission to define reliable, cost-effective, low-power, wireless networked, monitoring and control products, based on an open global standard. The Alliance provides the upper layer stack which includes the network, application as well as security layer. ZigBee version 0.8, which is based on IEEE 802.15.4, has flexible data transfer which is key-value and message based. It is also low cost and low powered with a low duty cycle. ZigBee version 1.0 was released on 14th December 2004. However the specifications are only available to members of the ZigBee Alliance, with public availability estimated at around mid-2005. Work on ZigBee 1.1 is ongoing.

ZigBee's short range of around 100m is suitable for many applications such as Home Automation Networks, Home Security Networks, Industrial Control Networks, interactive toys, remote monitoring, computer peripherals and so on. Table 1 compares ZigBee with some other protocols.

Table 1: ZigBee and other protocols

ZigBee is based on IEEE 802.15.4 which is suitable for low rate wireless personal area networks (WPAN) with three data speeds of 20, 40 and 250 kbps. IEEE 802.15.4 specifies only the MAC and Physical Layer. The radio has 3 frequency bands of operation (as seen in Table 1) and uses direct sequence spread spectrum. The MAC layer uses CSMA-CA with a maximum frame length of 127 bytes with data verification using 16-bit CRC.

The different ZigBee applications chosen would affect the choice of network topology. For example, a point to point topology can be used in applications like door bells, garage gates, toys, etc. A star topology can be used in applications like lighting and heating. A cluster tree topology can be used for site security and tracking. On the other hand, wired integration can be used for computer networks.

A typical ZigBee PAN model consists of Full Function Device (FFD), Reduced Function Device (RFD), Coordinator and Router. The Coordinator and Router are also usually FFDs. The Coordinator supports all services with the capability of peer-to-peer connection and is responsible for forming the network, i.e. it decides whether to accept or reject new network members. The Router is similar to the coordinator, except it also functions in joining different networks together. Routers and coordinators are therefore always turned on. A FFD can be a Coordinator, Router or an end device that supports majority of the services. RFD are end devices that automatically detect nearby coordinators to communicate through them.

Furthermore, there are also two types of networks available which are the beacon-enabled and the non-beacon networks. In a beacon-enabled network, the coordinator periodically transmits a signal that an end device uses in order to associate, join and synchronize data transfer. There is also optional support for guaranteed latency with slotted CSMA-CA to synchronize the slots. In a non-beacon network, the Coordinator also periodically transmits a signal, but this is only to disclose its presence and for the end devices to detect the coordinator. These devices communicate by sending data request and acknowledgement signals. Devices must be ready at all times to support peer-to-peer communications. Data would be sent from one device to another with the receiver acknowledging receipt. The usage of peer-to-peer communications is of course application dependent.

As mentioned earlier, ZigBee supports two types of data format which are key-value and message-based. Key-value is usually a generic command-response mechanism such as Get an Event and then Set according to the event. For example, when a user switches on a button, it signals to a chip that then turns on a light bulb. On the other hand, the message-based format supports straight binary data transfer. The data size is limited by the frame size supported.

ZigBee also has security features. It leverages the security model of the IEEE 802.15.4 MAC sub-layer which specifies four security services. They are access control, data encryption, frame integrity and a sequential freshness value used to reject data frames that have been replayed.

In conclusion, IEEE 802.15.4 is still a new standard that needs to pass through the various cycles of analysis by technology critics before establishing its own place in the industry. ZigBee has the potential to unify methods of data communication for sensors, actuators, appliances, asset tracking devices and so on. It offers the means to build a reliable and affordable network backbone that takes advantage of battery-powered devices communicating at low data rates. It can potentially create a whole new ecosystem of interconnected home appliances, light and climate control systems, and security and sensor sub-networks.

Thanks to Alex Lee for initial version of the page and compilation of resources Click here to return to Daniel Wong's home page click here to return to Daniel Wong's wireless resources page Click here for info on the book Wireless Internet Telecommunications Last modified 03-Feb-2006 07:06 PM